Many industrial engineers, such as in the lithographic industry, electron beam microscopy and space applications, deal with accurate positioning systems. Vibrations and other types of mechanical disturbances in such machines easily show up as a major factor in limiting the achievable accuracy, hence requiring significant reduction. Precise reproduction of the features that continue to get smaller requires good isolation from the environment while productivity concerns driven by market requirements require faster motion. Such demands impose special constraints on a vibration isolation design. Since, in many cases, the structural design of the isolated mass provides little inherent vibration isolation, and passive means provide insufficient isolation over the full required bandwidth, active means are often utilized to provide vibration control. In such applications, high-precision vibration isolation of a large payload with a high mass often requires vacuum compatibility, a contactless structure, high force density and low stiffness.
Air-based solutions are commonly used for actively isolating and controlling vibrations and other types of mechanical disturbances. In most lithographic applications, so-called air mounts are used, which are supplemented by electromechanical Lorentz-actuators providing stability control. A control valve regulates the flow of compressed air into a large air tank acting as a pneumatic spring. Unlike steel coil springs, the resonant frequency of this system is nearly independent of the mass of the payload, and the height control valve regulates the operating height. This provides gravity compensation and spring stiffness, where Lorentz actuators ensure stability and accurate positioning in all degrees of freedom.
The isolation bandwidth of the currently often used pneumatic isolators is generally limited. As a result the vibrations at elevated frequencies are not properly extinguished, which limits the performance of the machine being isolated. Furthermore, air bearings are only suitable for vacuum conditions if significant structural changes are applied which may adversely affect their performance.
Magnet-based vibration isolation systems are increasingly considered to be a feasible alternative for the passive or pneumatic vibration isolation systems. They offer distinct features such as being clean, noiseless and vibration and maintenance free. For these reasons they are increasingly being considered for use in vibration isolation applications. Examples of magnet-based vibration isolators are, e.g., found in U.S. Pat. No. 6,307,285
In ‘Zero-stiffness magnetic springs for active vibration isolation’ by Robertson et al., a permanent magnet system is used for obtaining a low stiffness vibration isolation system. This contactless magnetic spring uses attracting magnetic forces from the magnet above the load structure (negative spring, vertically unstable) and repelling magnetic forces from the bottom side (positive spring, vertically stable). These magnetic forces are oriented mainly along the axis of magnetization of the permanent magnets. The resulting vertical magnetic force compensates for the gravity force which is pulling down the load structure. Because the negative spring on the top and the positive spring at the bottom are placed in a parallel configuration, their respective spring stiffnesses add up and the resulting stiffness is near zero. This gives a vibration isolation system with a low natural resonance frequency to provide enhanced vibration isolation. It is, however a disadvantage of this magnetic spring type that the low spring stiffness is very local and as a result the spring stiffness varies significantly with the position. This allows for small movements only when low and constant spring stiffness is required or it requires significant control efforts for stabilization and isolation over a larger movement range. Furthermore, such a double-sided topology requires a ‘sandwiched’ construction which may prove to be disadvantageous if the vertical spring force exerted by such a magnetic spring has to be led around the spring by mechanical means.